This invention relates to fixed wing aircraft and, more particularly, to the configuration and method for controlling the flight of aircraft when the primary lifting surface is in deep stall.
It has generally been considered undesirable in normal operation of fixed wing aircraft to operate in deep stall. This condition occurs when the primary lifting surface, the wing, is at so positive an angle relative to the oncoming air flow, that flow line separation occurs along the entire upper wing surface and lifting force is lost. The result of such deep stall is generally an uncontrollable drop in attitude of the aircraft's nose section. Control is not regained except as air speed is increased by advancing engine thrust and by an accelerating drop in altitude.
Under the low speed and low altitude conditions of landing, where high wing lift is necessarily produced by flying close to wing stall conditions, the unintended incidence or inducement of deep stall might leave little time or room for recovery of control with damaging consequences. The seriousness of the problem is intensified when landings are required on short runways or over undeveloped landing areas.
Prior art has concerned itself with the role played by the tail surfaces, notably the stabilizer, in efficiently developing the highly positive angle of incidence required of the wing in low-speed flight conditions. The downward force generated at the rear of the fuselage by the stabilizer is used to rotate the aircraft to the wing's maximum unstalled angle of attack. On conventional airplanes having substantially horizontal stablizers, the downward thrust of the tail section is developed by upwardly tilting the elevator flaps hinged to the rear of the fixed stabilizer surface. However, in this slightly nose-up position of the airplane, the raised elevator flaps cause a downward thrust while, concurrently, the horizontal stabilizer section fixed in relation to the fuselage, generates increased lift (as compared to level flight) in opposition to the raised flaps. To overcome this inefficiency of opposed forces and to increase the down thrust while using smaller tail surfaces, fully tiltable stabilizers have been developed where the entire stabilizer surface rotates leading edge down, relative to the level flight axis of the fuselage. U.S. Pat. Nos. 2,563,757; 2,719,014; and 3,138,353 are illustrative of prior art utilizing tiltable stabilizer surfaces for the double purposes to more efficiently provide downward thrust at the rear of the fuselage and to simultaneously prevent the occurrence of deep stall in an aircraft flying at a low speeds.
Further, the desirability to use small or undeveloped landing zones has led to the development of aircraft capable of vertical takeoff and landing (VTOL). To achieve the VTOL feature and still retain relatively high performance in level flight, aircraft have been developed whereon the wing, or major portions thereof; engines; and, in some designs, the entire stabilizer surface are tiltable in the direction of flight. These elements are vertical for landing and takeoff and are horizontal in level flight as illustrated, for example, in U.S. Pat. No. 2,621,001. However, the stabilizer surface tilts with trailing edge down unlike the above-described fixed wing airplanes.
The VTOL airplane with tilting wing and engines suffers from the complexities of a dual purpose design and performance compromises which inevitably occur in attempting to satisfy two such diverse requirements as vertical and level flight. On the other hand, the more conventional fixed engine-fixed wing airplane with tiltable stabilizer surface preserves level flight performance and provides efficient aircraft control during low speed landings over shortened distances but does fall far short of effecting a vertical landing or nearly so. What is needed is a substantially conventional airplane having essentially normal level flight performance and combined with a capability of controlled landings which are vertical or substantially vertical.